The mineral industry consumes vast amounts of the total energy generated in the United States. And ore comminution by mechanical means consumes some 50% of the total energy required for mineral extraction. It has also been found that only about 1% of such energy of comminution is expended to generate new surfaces; the remainder is lost in frictional losses and heat. Thus a non-impact means of reduction has the potential for significant savings in energy. Capital costs may also be reduced.
Several nonmechanical methods have been suggested in the past but have been rejected for various reasons. The Snyder process, for example, comprised charging a coarse ore into a pressure chamber, pressurizing with a gas, and activating a quick-opening (15 millisecond) discharge valve which subjected the particles to a variety of impulse phenomena that caused reduction. Energy reduction in pilot plant studies did not justify further commercialization.
Primary reduction of large rocks by thermal stressing has been tried in the past (for example see U.S. Pat. No. 3,460,766, Sarapuu) but electrohydraulic crushing has received more attention. The latter technique involves the generation of a hydraulic shock wave of explosive intensity by a pulse discharge through water. It is, in truth, the nonmechanical compressive force which reduces the rock.
In the International Journal of Mineral Processing, volume 4, pages 33-38 (1977), Andres discusses a method for penetrating electrical discharge. He does not apply the discharge directly to the rock but again applies it to the liquid surrounding the rock resulting in attenuation of the electrical discharge energy. Two articles in Trans. Instn. Chem. Engrs. by Carley-MacCauly, et al. and Yigit, et al. (volume 44, page T395, 1966, and volume 47, page T332, 1969) discuss a similarmethod for fracturing brittle materials by means of a spark discharge through water.
In applying an electrical discharge through a nonconductor there are three regimes based on the duration of the discharge or pulse width. These regimes depend on separate mechanisms which yield significantly differing results.
The longest pulse width in the 0.1 second and longer range is in a thermal regime where the electrical discharge results in gross heating (exemplified by the Sarapuu patent). The intermediate impulse regime is characterized by pulse widths in the 100 microsecond to 100 millisecond range and results in a compressional force being applied over a period of time and fracture such as would be caused by mechanical impact.
The third regime comprises the shock discharge which is the subject of this invention. It occurs when the pulse width is in about the 10.sup.-3 -10.sup.-7 second range. The speed of the pressure wave away from the discharge exceeds the speed of sound thereby building up a pressure pulse through the sample. The rarefaction portion of this pressure pulse and reflected waves actually fracture the rock in tension along weak planes in the sample, generally along grain boundaries and mineralization veins.